The invention relates to multi-carrier digital transmission systems and has particular relevance to discrete multi-tone or orthogonal frequency division multiplexed systems for use over digital subscriber lines or radio broadcast systems.
Digital subscriber line technologies, commonly termed xDSL enable high-speed digital data to be transmitted down an ordinary phone line. The modulation scheme utilised for asynchronous DSL (ADSL) and proposed for very high speed DSL (VDSL) is discrete multi-tone modulation DMT. In this scheme, several carriers are quadrature amplitude modulated (QAM) at the same time and added together. Modulation can be achieved by performing an inverse fast Fourier transform (IFFT), with fast Fourier transform (FFT) used for demodulation. The output from one IFFT calculation is termed a discrete multi-tone symbol and is sent over the channel after digital to analogue conversion. A problem with normal telephone lines, which often comprise a simple twisted pair, is the frequency dependent attenuation and phase shift of a transmitted signal which result in the time dispersion or spread of the signal in time. This manifests itself as interference between adjacent symbols as one symbol is spread into a following symbol. The interference in one symbol is a combination of the interference due to a previously transmitted symbol, which is correctly termed the intersymbol interference ISI and the interference due to the symbol itself, or the intercarrier interference. For the purposes of this document, no distinction will be made between the sources of the interference and the term intersymbol interference or ISI will be used to designate the total effect of interference on a symbol.
ISI can be viewed as a transient or decaying xe2x80x98tailxe2x80x99 generated at the discontinuity where consecutive symbols meet. Conventionally, the effects of ISI are mitigated by providing a guard interval in front of each symbol. The guard time typically contains a cyclic extension of the symbol. Specifically, a copy of the end of each symbol is added to the beginning of the symbol in the form of a cyclic prefix. The carriers are continuous from the beginning of the cyclic prefix to the end of the symbol. Thus any interference will be generated at the discontinuity between the start of the cyclic prefix and the end of the previous symbol. The lengths of cyclic prefixes vary according to the application, but typically consist of no more than 10% of the symbol. Longer guard intervals are unfavourable because they introduce a bandwidth penalty. If the dispersion on the channel is not too severe, the ISI transient generated at the boundary between symbols will terminate within the cyclic prefix, leaving the subsequent symbol intact. However, the impulse response of the channel, including filters in the transmitter and receiver can be very long, and often exceed the guard interval. Residual intersymbol interference will then occur which can severely impair the quality of the received signals.
In the article xe2x80x9cResidual ISI Cancellation for OFDM with Applications to HDTV Broadcastingxe2x80x9d D. Kim and G. Stxc3xcber, IEEE Journal on Selected areas in Communications Vol. 16, No. 8, October 1998, a technique for cancellation of residual ISI is discussed. An algorithm is proposed for removing the interference generated between consecutive symbols transmitted on a channel. This includes the interference caused by the previous transmitted symbol, i.e. the inter-symbol interference (ISI), and the disturbance due to symbol itself, i.e. the inter-carrier interference (ICI). The determination of interference requires knowledge of the transmitted symbols. This is achieved by making decisions about the transmitted symbols utilising the received, decoded symbols that have been corrupted by the channel, with knowledge of the channel response. The estimated symbols are then converted back to the time domain using IFFT and the ISI determined and removed using the algorithm. The residual symbol is then reconverted the frequency domain using FFT and the decisions made. An iterative process then follows to remove the ICI. Since the decisions on the transmitted symbols may initially be erroneous, an iterative process is necessary to accurately determine the interference. This necessarily entails a large number of calculations, so that the process as a whole demands very high processing power.
Time domain equalizers TEQ are also used in the art to mitigate the ISI of symbols transmitted over a distorting channel. A time domain equalizer is a filter, generally a finite impulse response (FIR) filter and has the effect of shortening the impulse response of the channel. This can be achieved, for example, by cancelling the poles in the channel transfer function. By using a suitable algorithm, the channel impulse response can be made shorter than the cyclic prefix utilised. However, a drawback of TEQs is that both the noise and the signal are filtered. When a TEQ cancels the poles in the channel transfer function it will also attenuate some signal frequencies and amplify noise at other frequencies. The noise will leak into the side lobes of the fast Fourier transform in the receiver and degrade performance. Hence adapting the TEQ to minimise ISI will generally result in a sub-optimal signal to noise ratio.
There is thus a need for a system which reliably mitigates the effects of intersymbol interference while leaving signal information undisturbed but is simple enough to be employed for a wide range of applications.
In a multi-carrier transmission system wherein digital symbols are transmitted over a transmission medium to a receiver, intersymbol interference is compensated for by generating an estimate of the ISI tail and subtracting this from the received signals. This is achieved by determining an estimate of the transmitted symbols by filtering the received symbols and then generating a transient from the estimates of two consecutively transmitted symbols. Preferably, the difference between the estimates of two consecutively transmitted symbols is formed and the resulting difference symbol used to generate the transient which replicates the transient generated between the two consecutive symbols. The transient is then subtracted from the second of the received symbols to remove interference. The filter function used to estimate the transmitted symbols is substantially an inverse of the transmission medium transfer function for the carrier frequencies used. The transient is likewise generated by filtering using a filter having a transfer function substantially the same as the transmission medium transfer function. When utilising the difference symbol to generate the transient, the difference symbol is passed through the filter followed by a string of zeros.
Obtaining estimates of the transmitted symbols by subjecting the received symbols to a simple filter function minimises the number of calculations in the tail generation process. The number of calculations is still further reduced when a single difference symbol is utilised to generate the tail in place of two symbols. The processing power required for ISI cancellation is therefore small and acceptable for substantially all applications. Furthermore, depending on the accuracy of the filters and thus the estimates, ISI removal can be complete at all carrier frequencies, thus precluding the need for a guard interval.